They are built like bricks. Mines one of the older model but still in very good nick. I wouldn't worry about the older ones.
Do they use a IR sensor for the film like the D80 does?
I have an F100 and it's supposed to be an F5 in a smaller lighter body (a bit cheaper too)
It's great to handle - and the vertical grip MB-15 (I think) is also a good idea as it's is well balanced and still ends up smaller than the F5.